The properties of sugar transport by chicken embryo heart cells change during embryonic development. The capacity and kinetics of glucose transport change drastically during embryonic development, especially between the 5th and 10th day of incubation. Sorbitol, which doesn't enter adult heart cells, has been shown to freely permeate embryo heart cells. The immediate aim of this research program is to elucidate the mechanisms of these dramatic changes in sugar transport. The most likely explanations for the observed results are that the 5-day chick heart cell has a high passive permeability to glucose and sorbitol and later becomes impermeable to sorbitol and develops a facilitated transport system for glucose or that the 5-day heart cell has special high capacity transport systems for glucose and sorbitol that disappear or are modified as development proceeds. The aim of the proposed research is to investigate these possibilities.